Curable coating compositions, especially thermoset coatings, are widely used in the coatings art. They are often used for topcoats in the automotive and industrial coatings industry.
High-gloss and color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels. These coatings require an extremely high degree of clarity and a low degree of visual aberrations at the surface of the coating in order to achieve desired visual effects such as a high distinctness of image (DOI).
As a result, high-gloss and composite color-plus-clear coatings are susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out. It can be difficult to predict the degree of resistance to environmental etch that a high gloss or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as high-solids enamels, do not provide the desired level of resistance to environmental etch when used in high gloss coatings and color-plus-clear composite coatings.
Many compositions have been proposed for use as the clearcoat portion of color-plus-clear composite coating systems, such as polyurethanes, acid-epoxy systems and the like. However, many prior art systems suffer from disadvantages such as coatability problems, compatibility problems with the pigmented basecoat, and/or solubility problems. Moreover, very few one-pack coating compositions have been found that provide satisfactory resistance to environmental etch, especially in the demanding environment of automotive coatings.
It has been found that carbamate functional polymers such as those described in U.S. Pat. No. 5,356,669 can be used to provide coating compositions which exhibit significantly improved environmental etch resistance. Carbamate functional polymers have been used to provide commercially advantageous coatings compositions, especially as clearcoats in composite color-plus-clear coatings.
However, continual improvements in clearcoat coating compositions are always desirable. In particular, it would be advantageous to provide improvements with respect to environmental fallout, exposure to ultraviolet radiation emitted from sunlight, exposure to high relative humidity at high temperature, defects made by small, hard objects resulting in scratching and chipping, scratch and mar resistance, durability, and the like.
Balancing the competing needs of such performance characteristics continues to be challenging, especially with respect to automotive exterior coatings. For example, a harder film may provide a clearcoat that is more resistant to environmental etch, while resulting in a film that is less scratch resistant. A softer film may provide a more scratch resistant coating, with lessened etch resistance. Thus, an optimum balance of performance characteristics with respect to ever changing environmental factors is desirable.